workqueue: fix HIGHPRI handling in keep_working()

The policy function keep_working() didn't check GCWQ_HIGHPRI_PENDING
and could return %false with highpri work pending.  This could lead to
late execution of a highpri work which was delayed due to @max_active
throttling if other works are actively consuming CPU cycles.

For example, the following could happen.

1. Work W0 which burns CPU cycles.

2. Two works W1 and W2 are queued to a highpri wq w/ @max_active of 1.

3. W1 starts executing and W2 is put to delayed queue.  W0 and W1 are
   both runnable.

4. W1 finishes which puts W2 to pending queue but keep_working()
   incorrectly returns %false and the worker goes to sleep.

5. W0 finishes and W2 starts execution.

With this patch applied, W2 starts execution as soon as W1 finishes.

Signed-off-by: Tejun Heo <tj@kernel.org>